1. Field of the Invention
The present invention generally relates to an IP gateway apparatus. More specifically, the present invention is directed to a technique capable of securing qualities of voice packets, namely, capable of securing strictly high qualities required for delay time in IP gateway apparatuses capable of packeting voice data to send out voice data packets to IP networks.
2. Description of the Related Art
In VoIP (Voice over IP) services, improvements in voice qualities are required. That is, noise, fluctuations, and voice drops are wanted to be avoided. In the conventional techniques, voice packets are processed with priority over packets of other media (e.g., data), so that delays in voice packet arriving time can be avoided.
As a technique of a packet quality control method, Japanese Patent Application Laid-open No. Hei 4-306031 discloses “PACKET QUALITY CONTROL METHOD.” In this control technique, the packets are classified based upon a plurality of quality classes in accordance with the allowable delay time and the allowable discard rate. While the threshold values of the traffics within the system are set with respect to these quality classes, a quality control is carried out in such a manner that when the packets exceed the relevant threshold value, the priority order for transmitting the packets is determined based upon the quality classes.
In the above-explained conventional technique, while either the threshold value of the traffic within the system or the length of the line sending queue is compared with the threshold values of the respective quality classes, the priority order of the packet transmission is controlled. Otherwise, the packets are selectively discarded. In the conventional technique, the priority order in the case that the packeted PCM (pulse code modulation) signals are transmitted to the IP network is merely controlled in order to guarantee the qualities. However, this conventional packet quality control method does not solve the problem of delays occurred in the packeting process operation per se. As a result, in the above-explained prior art technique, there are possibilities that the delays occur in the packet processing operation within the system. In other words, deteriorations eventually occur in the delay time qualities, and therefore, the voice qualities are not secured.
FIG. 14 shows a structural diagram of a conventional IP gateway apparatus. FIG. 15 is an explanatory diagram for explaining a problem of the conventional IP gateway apparatus. In FIG. 14, both a CODEC processing unit and a MODEM processing unit packet a PCM signal received by a PSTN interface unit to produce either a voice packet (sound packet) or a data packet. Both a voice packet deriving unit and a data packet deriving unit derive either the voice packet or the data packet outputted from either the CODEC processing unit or the MODEM processing unit in response to a packet deriving instruction issued from a control unit.
In the conventional IP gateway apparatus, the following operations may constitute factors for packet processing delays. That is, while the CODEC processing unit executes the packeting process operations for the plural channels in the parallel manner, this CODEC processing unit outputs the data packets of the plural channels at the timing which is under asynchronous condition. Furthermore, when a packet is transmitted to the IP network, either the voice packet deriving unit or the data packet deriving unit derives this packet from either the CODEC processing unit or the MODEM processing unit by way of the polling process operation. In this case, the time period of this polling process operation is not synchronized with the output timing of the packet. In other words, the packeting process operation is carried out in asynchronism with the packet deriving process operation. As a consequence, as to a certain voice, there is such a possibility that a maximum delay is produced which is equal to the time period of the packeting process operation (see problem No. 1 shown in FIG. 15). Assuming now that even if such an arrangement is employed in which the CPU (Central Processing Unit) executes the IP packeting process operation by an interruption made by the DSP (Data Signal Processor), since the interruption made by the DSP is uncertain, the interruption process operations are concentrated within a given time duration. Otherwise, there is an overhead required to perform the interruption process operations. As a result, there is such a chance that the delays are produced in the real-time processing operation (see problem No. 1 shown in FIG. 15).
Furthermore, in the IP gateway apparatus, not only voice calls, but also data calls are mixed with each other. When the voice packets are sent out to the IP network, since the data packets are interrupted among the voice packets, there may be fluctuation of voice packets (see problem No. 2 indicated in FIG. 15).
The ITU G.114 recommendation requests that the delay time should be suppressed within 150 milliseconds. While the above-explained process delays and fluctuations may give large adverse influences to the voice qualities, if the delays and the fluctuations are increased, then the end users must listen to voices (sound) having deteriorated qualities. To avoid this problem, on the reception side of the voice packets, both the delays and the fluctuations are tried to be solved by increasing the fluctuation absorbing buffers. However, even when the fluctuation absorption buffer is increased, the problems of both the delays and the fluctuations cannot be solved, and therefore, the delayed packets should be discarded. In this case, the qualities of the voice could not be maintained at the desirable levels. As a consequence, the following requests are made on the packet transmission side. That is, the delays produced in the packet transmission process operation are to be shortened, and also the fluctuations are to be prevented.